Based on my reading of the MSDN doc this should just result in returning the default value, not throwing an Exception.

"Returns the last element of an observable sequence that matches the predicate, or a default value if no value is found."

http://msdn.microsoft.com/en-us/library/hh228948(v=vs.103).aspx

It should return the last value that matches the predicate, but if nothing matches then it should return the default value.